/**
 * 合并两个排序的链表
 */

function mergeTwoLists(l1, l2) {
  if (!l1) return l2;
  if (!l2) return l1;

  let head;
  if (l1.val < l2.val) {
    head = l1;
    head.next = mergeTwoLists(l1.next, l2);
  } else {
    head = l2;
    head.next = mergeTwoLists(l1, l2.next);
  }
  return head;
}
